In recent years, ink-jet recording systems, which enable simple formation of images at low cost, have been used in various printing applications such as photography, various types of printing, and special printing such as marking and color filters. Specifically, by employing ink-jet recording apparatuses, which eject controlled minute dots of ink, ink-jet inks which have been improved in color reproduction, durability, and ejection adaptability, and specialized paper media which have been enhanced in color forming properties of colorants and surface glossiness, it has become possible to achieve image quality comparable to conventional silver halide photography. Image quality of the present ink-jet recording systems is enhanced only when an ink-jet recording apparatus, an ink-jet ink and a specialized paper medium are simultaneously improved.
Specifically the ultraviolet ray curable ink-jet system which cures the ink by cross-linkage of ultraviolet (UV) rays after image recording, requires no specific image receiving layer and it is possible to record onto various types of base materials (namely recording media), resulting in drawing attention in the market, and has also resulted in the study of wider application, (please refer, for example, to Patent Document 1). However, even if this system can record an image onto various base materials, there have been drawbacks such as insufficient adhesion to the base materials and insufficient physical properties of the ink layer.
In an ultraviolet ray (UV) curing method, since the reaction proceeds instantaneously under light irradiation whereby monomers are three-dimensionally cross-linked, shrinkage of the ink in curing and deterioration of ink layer flexibility results. As a result, easily caused tend to be problems such as decreased flexibility, resulting in cracks in the image area when the base material is sharply bent or cut. Further, in the UV curing method, mass decrease of the ink does not occur on the base material, and the ink layer remains in a thick state, resulting in undesirable decreased flexibility.
To overcome these problems, it has been considered to add a liquid exhibiting no reactivity such as a plasticizer or a solvent of relatively high boiling point in order to provide ink layer flexibility, however these materials are not preferable from the view point of being VOC-free (namely a volatile organic compound), and insufficient curing sensitivity.
At present, one of the most effective countermeasures is to decrease three-dimensional crosslink density, that is, to increase the ratio of mono-functional monomer having lower cross-linking points. A mono-functional monomer generally exhibits low viscosity, which low viscosity is preferable to obtain ejection suitability of the ink-jet ink. In addition, the low viscosity is effective in preventing wrinkling which is peculiar to UV curing. (Please refer, for example, to Patent Document 2.)
However, when the ratio of a mono-functional monomer in the ink is increased, curing reactivity is decreased and deterioration of curing sensitivity is generated. This results from the fewer reaction points, but in addition to that, there is the issue which makes the ink susceptible to reduced curing reaction inhibition in an oxygen ambience. Specifically in an ink-jet recording method employing small ink droplets to obtain a higher resolution image, the effect of decreased sensitivity due to oxygen inhibition is marked.
Improvement of the effect of curing reaction inhibition by oxygen is proposed to conduct the reaction under a nitrogen atmosphere. (Please refer, for example, to Patent Document 3.) Also proposed has been an attempt to decrease the volume of photo-polymerization initiator. However, in that method, the apparatus becomes more complex, resulting in major factor of higher cost. For example, in the case of a large serial printer, enclosing of the equipment in a nitrogen atmosphere is accomplished only with much difficulty.
Patent Document 1: European Patent 540,203 B2
Patent Document 2: Unexamined Japanese Patent Application Publication No. (hereinafter, referred to as JP-A) 2004-175906
Patent Document 3: U.S. Pat. No. 6,550,905 B1